Page 367 - Mechanical Engineers' Handbook (Volume 4)

356 Heat Pipes

generates the pulsating motion of liquid plugs and vapor bubbles in the system. In addition
to the pulsating motion observed in the system, the oscillating motions of liquid plugs and
vapor bubbles exist at the same time. For this reason, the PHP is sometimes called an
oscillating heat pipe (OHP). The oscillating motions in the PHP depend on the dimensions,
working fluids, operating temperature, surface conditions, heat flux and total heat load, ori-
/V , where V is the liquid volume
entation, turns, and, most importantly, the filled ratio, V l t l
occupied by the liquid in the system, and V is the total volume. 16,17 Utilizing phase-change
t
heat transfer and forced convection, the heat is transferred from the evaporating section to
the condensing section. Compared with a conventional heat pipe, the successful pulsating
heat pipe (PHP) has the following features: (1) there is a low-pressure drop in the working
fluid because most or all of working fluid does not flow through the wick structure; (2) the
PHP is very simple and the manufacturing cost be very low because no wick structures are
needed in most or all sections of the PHP; (3) the liquid pressure drop caused by the frictional
vapor flow can be significantly reduced because the vapor flow direction is the same as liquid
flow; (4) the thermally driven, pulsating flow inside the capillary tube will effectively produce
some ‘‘blank’’ surfaces that produce thin-film regions and significantly enhance evaporating
and condensing heat transfer; and (5) the heat added on the evaporating area can be distrib-
uted by the forced convection in addition to the phase-change heat transfer due to the os-
cillating motion in the capillary tube. Clearly, the PHP creates a potential to remove an
extra-high level of heat flux. On the other hand, the diameter of the pulsating heat pipe must
be small enough so that vapor plugs can be formed by the capillary action.

5.4 Micro Heat Pipes

In 1984, Cotter 18 first introduced the concept of very small ‘‘micro’’ heat pipes, which was
incorporated into semiconductor devices to promote more uniform temperature distributions
and improve thermal control. The micro heat pipe was defined as a heat pipe in which the
mean curvature of the liquid–vapor interface is comparable in magnitude to the reciprocal
of the hydraulic radius of the total flow channel. Based on this definition, the hydraulic
diameter of a typical micro heat pipe ranges from 10 to 500 m. The fundamental operating
principle of micro heat pipes is essentially the same as those occurring in relatively large
conventional heat pipes. A typical micro heat pipe shown in Fig. 14 is using the cornered
region to pump the condensate from the condenser to the evaporator. As heat is added on
the evaporating section, the liquid vaporizes and the vapor brings the heat through the adi-
abatic section to the condensing section, where the vapor condenses into the liquid and
releases the latent heat. The heat addition on the evaporating section causes the liquid to
recede into the cornered region and directly reduces the meniscus radius at the liquid–vapor
interface in the evaporator. This vaporization and condensation process causes the liquid–
vapor interface in the liquid arteries to change continually along the pipe and results in a
capillary pressure difference between the evaporator and condenser regions. This capillary
pressure difference promotes the flow of the working fluid from the condenser back to the
evaporator. As the size of the heat pipe decreases, however, the micro heat pipe may en-
counter the vapor continuum limitation. This limitation may prevent the micro heat pipe
from working under lower temperature. In addition to the vapor continuum limitation, the
micro heat pipe is also subject to the operating limits occurring in the conventional heat
pipe. Of those operating limits, the capillary limitation remains the most important for the
micro heat pipe. Micro heat pipes have been widely used in the electronics cooling.

5.5 Variable-Conductance Heat Pipes

For a typical conventional heat pipe, the operating temperature can be determined by the
heat-removal rate from the condenser. When the heat load increases, the temperature drop